1. Field of the Invention
The invention relates to an improved method for purifying water containing dissolved hydrocarbons and chemicals. The improvement implies that some of the most environment hostile, dissolvable components also can be separated. This may be achieved by minor modifications of the existing purification equipment, by using a condensate flow from the production system as extraction medium.
2. Description of the Related Art
There is a demand for cleaning processes which removes dissolved aromatics from produced water. Spillage to water has attracted increased attention, and new environmental requirements introduced may result in requirement for reinjection of produced water, if the methods for cleaning are not improved. For older oil fields with increasing water production, such requirements may ultimately result in close down. One single platform at the Statfjord field produces to-day 32,000 m3/d produced water.
Hydrocyclones are used for purifying water in a production plant for oil and gas. This technology meets the present cleaning requirements of 40 ppm for free oil in water, most often within good margins. Harmful effects of serious degree on the environment occur rarely at such low concentrations of spillage.
Hydrocyclones do not remove dissolved hydrocarbons from the water. Produced water contains also dissolved components, including aromatics such as benzene, toluene and xylene (BTX), naphthalene, phenantrene (NPD), and polyaromatic hydrocarbons (PAH). Such compounds are destroyed slowly, and accumulates in the food chain. Certain chemicals employed in oil production may have corresponding properties.
International Patent Application No. PCT/NO97/00060 discloses a system for removal of water soluble hydrocarbons in a water flow. This process is based on injection and fine dispersion of natural gas in a water flow at pressure and temperature conditions permitting the gas to function as solvent for hydrocarbons dissolved in the water. The dispersed gas bubbles expand when absorbing the water solvent hydrocarbons and achieve a reduced density compared to the water, which makes them easy to be remove, for example by applying hydrocyclones. This method will, however, not be sufficiently effective in practice.
The above referenced International Application No. PCT/NO97/00060 claims to relate to a method for extraction of dissolved and dispersed hydrocarbons from water. Extraction of dissolved hydrocarbons is mentioned, but the technical description does not cover the conditions which are necessary to achieve this effect.
In reality this publication discloses only unit operations for removal of dispersed oil from water, and is not, as such, suitable to solve the same problem as the present invention:
The description of separation is focused on drops of dispersed oil, with emphasis on the fact that reduced density and increased size of the oil drops increase the degree of separation.
That the use of “liquefied natural gas” in claim 1 has a function in preference of other hydrocarbons in fluid form, and is linked to low density of such.
Extraction of aromatics from the water phase is primarily mentioned in the description as an additional effect. Neither the patent claims nor the description disclose the required conditions to be met, such as area of contact, time of retention or concentrations of the aromatics.
The conditions which are described as necessary in order to increase separation of dispersed oil, is neither sufficient nor necessary in order to provide separation of dissolved components. The general statement that liquefied hydrocarbons may be used for extraction of liquefied compounds in water, may be considered as known, even if the hydrocarbon phase is “liquefied natural gas”.
Application of “liquified natural gas” implies a negative effect on existing hydrocyclone systems, the applicant specifying that “Hydrocyclones operates at pressure and temperature such that the gas is in a liquefied form at the cyclones overflow”. It is well known within the industry that cooling and changes in pressure require equipment which may result in heavy deterioration of the purifying process for produced water. Hence, it is not a very desirably solution to change the pressure and temperature of the fluids to be separated.
PCT/NO97/00060 describes in principle application of “liquefied natural gas” for improving separation of dispersed oil in hydrocyclone based systems. A major feature in the Patent Specification is that increase of differences in density between the oil phase and the water phase is credited as providing a major effect. The Patent Specification does also specify that dissolved components are removed during the process, but the specification lacks a description on how this may be achieved.